1. Field of the Invention
The field of the invention is that of optical transmission networks and to be more precise that of networks utilizing wavelength division multiplexing. It relates primarily to a method of assigning to signals to be transmitted carrier frequencies that can be used in this kind of network.
2. Description of the Prior Art
Optical networks essentially comprise nodes interconnected by optical links. A node is often connected to a plurality of other nodes, in which case it incorporates routing functions for selectively switching signals received from upstream links to different downstream links as a function of the respective destinations of the signals. Some nodes have access functions for adding signals to the network and/or dropping signals from the network.
In wavelength division multiplex (WDM) networks, the signals conveyed by the links are WDM signals comprising a plurality of components carried by respective different optical frequencies. A WDM signal is therefore formed of a combination of optical signals each consisting of an optical carrier wave modulated as a function of the information to be sent. Each optical frequency (or the corresponding wavelength) of a carrier wave therefore defines a corresponding WDM channel. In the remainder of this description, the channels are referred to interchangeably by their optical frequencies f or by their wavelengths λ, which are related by the equation f=c/λ in which c is the velocity of light in a vacuum.
In WDM networks, a first option is for the nodes to be equipped with regenerator devices operating separately on each channel of the wavelength division multiplexes received to carry out reshaping and resynchronization. The network is then known as an opaque network and has the advantage of providing a minimum transmission quality fixed for all the routes that the various channels may take. On the other hand, the regenerators affect the cost of the network, which increases as the number of WDM channels increases.
A less costly solution is to design a network with no individual channel regenerator devices. This kind of network is referred to as transparent but may nevertheless include optical amplifiers adapted to amplify simultaneously the channels of the wavelength division multiplexes transmitted. There is additionally the compromise of a hybrid network in which only some links are provided with regenerators.
In a transparent network (or in the transparent portion of a hybrid network), the links between two nodes, whether direct or indirect via other nodes, are themselves transparent and must on average be lower than in an opaque network because, despite the possible inclusion of amplifiers, the signals are degraded to an extent that increases with the transmission distance. A transparent network therefore offers less flexibility of design and use.
Furthermore, it has been observed that the transmission quality of an optical signal depends among other things on the optical frequency of the wave carrying the signal.
For example, in the C band of wavelengths from 1 530 nm to 1 560 nm, the lower wavelengths are found to be less “efficient”, i.e. achieve transmission qualities that on average are worse than those achieved with higher wavelengths. This is because, at lower wavelengths, losses in the fibers in particular are higher, more noise is introduced by conventional amplifiers, and nonlinear effects are aggravated.
This phenomenon remains marginal provided that a transparent network uses carrier frequencies belonging to a relatively narrow band of frequencies. However, it is not without consequences as the band of frequencies that can be used becomes ever wider.
The simplest way to take account of this phenomenon is to rate the network (link wavelengths, amplifier gains) as a function of the least efficient carrier frequencies. However, this means that the network is rated higher than it need be for other frequencies.
Various compensation measures might also be envisaged, such as the use of dedicated amplifiers of higher gain at low wavelengths, or controlling chromatic dispersion with a view to equalizing the transmission performance of the various wavelengths of the band used. These solutions always lead to an additional cost or to less than optimum use of the resources of the network, however.
Accordingly, an object of the invention is to improve the use of the resources of a transparent or partially transparent network.
On examining the operating conditions of transparent networks in more detail, it emerges that the signals to be transmitted do not impose uniform transmission constraints. For example, the transmission distance without regeneration varies from one signal to another according to the destination of the signal and the route chosen to reach it. Similarly, in some networks there may be a plurality of transmission data rates, depending on the signals concerned. Then again, there may be different classes of service that can be assigned to the signals, those classes imposing respective different maximum permissible values of the error rates affecting the signals received after transmission.
Accordingly, in a transparent network, it is found, firstly, that the transmission constraints vary as a function of the signals and, secondly, that the transmission quality of an optical signal depends on the frequency of the carrier wave. Rather than seeking to compensate the differing efficiencies of the carrier waves according to their frequencies, the invention seeks a more economic approach that, to the contrary, matches the varying efficiencies of the carrier frequencies to the varying constraints of the signals to be transmitted, i.e. establishes a rule for assigning the more efficient frequencies to the signals with the more severe constraints, and vice versa.
This kind of assignment rule can be generated if it is possible to define beforehand a measurement of the efficiency of the frequencies and a measurement of the “constraining” character of the signals. To define these concepts, it must be possible to attach them to measurable parameters operative in transmission over transparent links.
One decisive parameter is the transmission quality of a signal, which may be measured directly by determining the error rate of a signal received after transmission. This quality is conditioned firstly by physical parameters associated with the link, such as its length, the type of fiber used, and the amplifier characteristics. It depends, secondly, on parameters associated with the physical properties of the signal. Those physical properties are conditioned by the spectrum of the signal, and thus by its carrier frequency and its data rate, as already mentioned, and additionally by its modulation format. The transmission quality of a signal also depends on its environment, i.e. the presence and the nature of other signals transmitted simultaneously on nearby frequencies.
With regard to the efficiency of a given frequency, it can be evaluated from the mean value of the error rate values of the signals carried by that frequency as received after transmission in the network concerned.
As for the constraining character of a signal, this may be the result of a plurality of parameters linked to each signal to be transmitted and referred to hereinafter as “transmission constraint parameters”. The transmission distance, the data rate, and the maximum error rate are examples of such parameters. The resulting “constraint value” is therefore a function of the various transmission constraint parameters that can be assigned to a signal.
When the efficiency of a frequency has been defined, and likewise a function for evaluating the constraint values of the signals, it becomes possible to establish a one-to-one correspondence between each frequency and a constraint value that the frequency concerned might assume. In practice, however, to allow some flexibility in assigning an appropriate frequency to any signal, it is necessary to construct a limited predefined number N of classes or sets of frequencies such that each class groups together the frequencies whose efficiencies are within a given range of values, and thus whose mean error rates are also in a given range of values. The function of the constraint parameters mentioned hereinabove can therefore be seen as a staged function able to assume N distinct values, referred to as constraint values, associated with the N sets of frequencies.
The foregoing considerations make it possible to define more precisely the method of the invention for assigning the carrier frequencies to the signals.